1. Field of the Invention
The present invention generally relates to ink jet printhead apparatus and, more particularly, to a dual element switched digital drive system for piezoelectrically driving an ink jet printhead.
2. Description of Related Art
A piezoelectrically actuated ink jet printhead is a relatively small device used to selectively eject tiny ink droplets onto a paper sheet operatively fed through a printer, in which the printhead is incorporated, to thereby form from the ejected ink droplets selected text and/or graphics on the sheet. In one representative configuration thereof, an ink jet printhead has a horizontally spaced parallel array of internal ink-receiving channels. These internal channels are covered at their front ends by a plate member through which a spaced series of small ink discharge orifices are formed. Each channel opens outwardly through a different one of the spaced orifices.
A spaced series of internal piezoelectric wall portions of the printhead body separate and laterally bound the channels along their lengths. To eject an ink droplet through a selected one of the discharge orifices, the two printhead sidewall portions that laterally bound the channel associated with the selected orifice are piezoelectrically deflected into the channel and then returned to their normal undeflected positions. The driven inward deflection of the opposite channel wall portions increases the pressure of the ink within the channel sufficiently to force a small quantity of ink, in droplet form, outwardly through the discharge orifice.
According to a recently proposed drive method for this type of ink jet printhead, top sides of the internal channel dividing wall portions are commonly connected to ground, and the bottom sides of the wall portions are individually connected to a series of electrical actuating leads. Each of these leads, in turn, is connected to a drive system operable to selectively impart to the lead an electrical waveform that sequentially changes (1) from ground to a first driving polarity, (2) from the first polarity to the opposite polarity, and (3) from the opposite polarity back to ground.
When this electrical waveform is imparted to a piezoelectric channel wall portion bounding one side of a selected, and a second electrical waveform of opposite polarity sequence is simultaneously imparted (via another one of the actuating leads) to the opposite piezoelectric channel wall portion, the opposite channel wall portions, by piezoelectrical action, are sequentially deflected (1) outwardly away from the channel that they laterally bound, (2) into the channel to discharge an ink droplet therefrom, and (3) back to their starting or "neutral" positions.
Both analog and digital type drive systems have been suggested for providing the above drive method in an ink jet printhead. As analog type drive systems which utilize analog circuitry, for example, operational amplifiers (or "op-amps"), to deliver the desired electrical waveform to the ink jet printhead are linear in nature, however, such drive systems tend to produce unacceptably high levels of power dissipation and have, therefore, proven inefficient in use. Furthermore, such analog type drive systems require excessive space on the printhead, thereby adversely affecting driver density for the printhead.
Proposed digital type drive systems, on the other hand, while avoiding the aforementioned deficiencies relating to power dissipation, also have several built-in limitations and disadvantages. More specifically, such digital type drive systems utilize switching type circuits, also referred to as "digital drivers". Since the proposed drive method requires a bipolar voltage waveform with an active return to ground, digital type drive systems have heretofore required three separate drivers--one for each of the three channel wall drive portions described above. This requirement substantially increases the complexity of the drive system, thereby undesirably increasing its overall cost. Additionally, it undesirably increases the overall space requirement for the drive system.
It can be readily seen from the foregoing that it would be desirable to provide an improved ink jet printhead drive system that eliminates, or at least substantially reduces, the above-mentioned limitations and disadvantages associated with the drive systems described above. It is accordingly an object of the present invention to provide such an improved ink jet printhead drive system.